The present invention generally relates to doctor blade assemblies which define an enclosed fluid reservoir for applying fluid to a rotating roller, and more particularly relates to a self-lubricating end seal used to seal the opposite ends of the fluid reservoir, the top and bottom of which are enclosed by a pair of doctor blades which engage with the roller.
Enclosed doctor blade assemblies are used extensively in machinery utilizing a rotating roller that picks up fluid from a reservoir and deposits the fluid onto another surface located opposite the doctor blade assembly. Examples of such machinery include rotary printing units such as flexographic printing machines. Such enclosed doctor blade assemblies can also be utilized for the application of varnish, adhesives and various specialty coatings, for example. In a flexographic printing station, the enclosed doctor blade assembly delivers ink to the surface of an engraved roller, often referred to as the anilox roll. The surface of the anilox roller contains engraved microscopic cells that carry and deliver a pre-determined quantity of ink to the surface of the printing plate.
The enclosed doctor blade assembly is intended to form an intimate seal with the surface of the anilox roller. The top and bottom, longitudinal surfaces of the assembly are sealed by means of two doctor blades. The doctor blades are mounted to the reservoir and positioned in parallel, spaced relation to each other and are directed at strategic angles to engage the free edges of the doctor blades with the surface of the anilox roller. The doctor blades extend the full length of the anilox roller. The function of each doctor blade is determined by the rotational direction of the anilox roller with one blade metering the ink or other fluid from the surface of the anilox roller while the other blade simply acts in an ink containment role, holding ink within the reservoir (see, for example, U.S. Pat. No. 5,125,341). In such enclosed doctor blade assemblies, the reservoir and doctor blades contain the fluid except at the opposite ends thereof which are open and must be closed with specially configured seals to completely enclose the reservoir and ensure the fluids (e.g., printing ink and cleaning solutions) do not unintentionally leak from the enclosed fluid reservoir. The part of the seal that faces the radial surface of the anilox roller comes into direct contact with the surface of the anilox roller when the enclosed doctor blade assembly is put into operation. The remaining surfaces of the seal are in contact with the inner surfaces of the two doctor blades and the frame forming the reservoir of the enclosed doctor blade assembly.
Traditional doctor blade assembly end seals are manufactured from compressible foam and rubber materials which are very susceptible to uncontrolled deformation and dislodgement from the ideal operating position relative to the surface of the anilox roller, particularly when exposed to changes in the internal operating pressure of the enclosed fluid reservoir during normal operation, as well as the inherent mechanical drag applied by the rotation of the anilox roller, especially at elevated press speeds. Once dislodged from the correct operating position the normal life expectancy of the seal is shortened considerably and ink leakage starts almost immediately. Even if such a seal is not completely dislodged from the fluid reservoir frame, even minor unintended deformation or seal movement within the frame immediately leads to premature wear and some degree of unwanted leaking of the ink or other fluid from within the enclosed ink reservoir. Press operators are then forced to stop the machine production to change/replace the worn or dislodged seals. There is also excessive cost associated with the wasted ink as well as additional cleaning of the machine and various press components that are exposed to the leaking ink.
The surface of the anilox roller is quite hard (e.g., 1250-1300 Hv (Vickers scale; equivalent to Shore C +70) and abrasive due to the fluid-holding cells engraved into it which act to coat fluid onto the roller surface as it rotates through the fluid reservoir. As such, the end seals are exposed to significant abrasive wear as the anilox roller rotates, particularly at very high speeds which result in a proportional increase in the COF (coefficient of friction) and mechanical stress applied to the seals. New servo-drive, gearless flexographic presses have dramatically increased the machine production speeds that can be obtained. As the rotational speed of the anilox roller is increased, there is a proportional decrease in the life-expectancy of the surface of the seals in contact with it. Thus, as machine speeds have continued to increase, the industry has seen the prior art seals wearing out or otherwise failing faster than ever before.
Besides being subjected to abrasive wear, the end seals are also exposed to various levels of hydraulic pressure applied by the reservoir fluids (e.g., the printing ink and cleaning solutions) that are pumped into and out of the reservoir during normal operation of the press. The wear rate of the surface of the seals in contact with the anilox roller is thus directionally proportional to the anilox roller surface abrasiveness, hydraulic pressure applied by the reservoir fluids, and the speed at which the anilox roller is turning (rpm's). This rapid wear of the seals results in a considerable decrease in productivity due to the press operator having to frequently stop the printing press to replace worn, dislodged or leaking end seals in each of the print stations. A typical gearless flexographic press will have between eight and ten print stations having a pair of seals in each. In addition, modern servo presses are typically equipped with an automatic wash-up feature that facilitates very quick transition to the next print job. During the switch from one print job to the next the automatic wash-up cycle is initiated and any ink that remains in the enclosed ink reservoir from the completed job is extracted using suction and then charged (pumped) with cleaning solution. This cleaning cycle exposes the seals to varying degrees of negative followed by positive hydraulic pressure as ink is removed and cleaning solution is pumped/sprayed through nozzles and circulated within the reservoir of the enclosed doctor blade assembly. During the ink extraction stage and delivery of cleaning solution to the enclosed ink reservoir, a significant change in the internal operating pressure of the ink reservoir occurs due to the suction required to remove the left-over ink as well as the cleaning solution once the cleaning cycle is complete. This change in internal pressure within the enclosed ink reservoir has been known to dislodge one or both the end seals from their ideal operating position, leaving the seals incorrectly oriented relative to the surface of the anilox roller. If the machine is then operated with the end seals in an incorrect orientation relative to the curved surface of the anilox roller, the end seal wears rapidly (similar to having unbalanced tires on a car) which, if not caught by the operator, results in a loss of intimate contact with the surface of the anilox roller which in turn allows ink to enter the area where the seal makes contact with the surface of the anilox roller. Once the printing ink enters this area (between the surface of the seal facing the roller and the anilox roller) the ink starts to dry which then adds to the rate of abrasive wear on the seal. Compounding the problem is that the new gearless press technology runs at 2-3 times the production speed of conventional gearless presses. As such, there is a significant increase in the level of mechanical stress applied to the end seals in the rotary direction where it makes contact with the surface of the anilox roller.
There thus remains a strong need in the industry for enclosed doctor blade end seals which are much more durable and failure resistant than the end seals which have been used to date.